Napoli fans are said to have attacked a north London restaurant favoured by Arsenal supporters before the clubs' Tuesday evening Champions League clash.
Piebury Corner near the Emirates Stadium had to close after dozens of thugs tore through the building smashing windows, breaking chairs and swinging belts.
A man in his 40s was taken to Whittington Hospital in north London with cuts to his head, though a London Ambulance Service spokesman said his injuries were not thought to be serious.
Witness Emily Gould, who works at the eatery, said around 50 customers were inside the premises, on Holloway Road, when a 30-strong mob of rival supporters stormed the building prior to kick-off.
She said: "I think they were Italian men with Napoli colours and scarves tied around their faces. They were shouting something in Italian, chanting and causing trouble down the street.
"Then then came in to the shop. I was terrified. They started swinging belts around. The customers were very frightened."
Napoli fans at Arsenal
Ms Gould said many of those already inside Piebury Corner ducked for cover as the rival fans entered. She said one man had to be taken to hospital with what she believed was a deep gash to the head after he was struck by a swinging belt buckle.
"There is blood all over the floor," the employee said.
"We have been told by police we cannot clear up yet because of the investigation. There is a lot of damage to the shop. The tables and chairs are wrecked."
The Metropolitan Police described the incident as "football-related". A spokesman said a man had been arrested in the area for a public order offence, unconnected to the damage at Piebury Corner.
Arsenal beat Napoli 2-0
Piebury Corner owner Paul Campbell said the attack was "completely unprovoked".
He said: "We have had Napoli fans inside all day today, they have been having a good time and not bothering anybody at all. They have been no problem.
"Then we had a minority of them come in, being violent, picking up chairs and tables. There were three girls sat near me and these thugs just came in and grabbed the chairs from under the girls.
"It was completely unprovoked, I think they just picked us at random because they saw a busy restaurant and they wanted to fight someone - anyone."
Mr Campbell said he had no reason to suspect any trouble from Napoli fans before kick-off.
He said: "We are popular with home fans and the travelling European fans too. This just came from nowhere."
Last year, Tottenham fans were attacked by Italian thugs in Rome ahead of their Europa League tie with Lazio.
Arsenal defeated Napoli 2-0 during the sides' clash.